1. Field of the Invention
The present invention relates to rail car braking systems and, more specifically, to a system for providing an accurate brake cylinder reference pressure and improved pressure regulation.
2. Description of the Related Art
The quick service limiting valve of an Association of American Railroads (AAR) type braking system controls the supply of brake pipe air to the brake cylinder during quick service operations. Recent advancements in quick service limiting valves include modifications to the quick service limiting valve that assist in the replenishment of brake cylinder pressure that may be lost, such as through leakage. For example, one approach is to use a brake cylinder reference pressure based on the actual brake cylinder pressure that pressurizes one side of the diaphragm in the quick service limiting valve so that if brake cylinder pressure leaks below a threshold set by the pressure on the opposing side of the diaphragm, the resulting force imbalance will cause movement that opens communication between the brake pipe and the brake cylinder, thus replenishing any lost brake cylinder pressure, until the force balance is re-established.
In order for this arrangement to work effectively, however, the brake cylinder reference pressure must accurately represent the desired target brake cylinder pressure in all possible scenarios. Current approaches work very well in the situation where the brake cylinder develops a leak after it has been fully pressurized to the desired level. These approaches are not as effective when the brake cylinder has a pre-existing leak such that the brake cylinder leaks while the brake cylinder pressure is being charged. In this situation, the actual brake cylinder pressure is not likely to achieve the desired pressure value. As a consequence, the brake cylinder maintaining reference pressure will not achieve the desired target value and there will not be a desired force imbalance across the quick service limiting valve diaphragm that opens communication between the brake pipe and the brake cylinder to replenish lost brake cylinder pressure.